Diabetes is categorized into Type 1 diabetes (insulin-dependent diabetes) and Type 2 diabetes (non-insulin-dependent diabetes), and borderline type diabetes (glucose tolerance disorders) has also attracted attention as a pre-diabetic condition in recent years. Type 1 diabetes is characterized by a partial or complete inability to produce insulin, which is a blood glucose regulating hormone. Type 2 diabetes is characterized by induced peripheral insulin resistance and impaired insulin secretion. Borderline type diabetes is a pathological condition exhibiting impaired glucose tolerance (IGT) or impaired fasting glucose (IFG), associated with a risk of developing Type 2 diabetes or diabetes complications.
Diabetes is caused by several predisposing factors. It is a disease characterized by high glucose levels in blood plasma in fasting and postprandial states or during an oral glucose tolerance test or by chronic hyperglycemia, in general. Controlling chronic hyperglycemia is essential in clinical management and treatment of diabetes. In particular, reduced insulin secretion from beta cells of the pancreas can induce an abrupt increase in postprandial blood glucose levels in Type 2 diabetes or borderline type diabetes. An international large-scale clinical trial has revealed that it is essential to control postprandial hyperglycemia in impaired glucose tolerance for suppressing the development and progress of not only diabetes but also hypertension and cardiovascular diseases (JAMA, 290, 486-494 (2003) (Non-Patent Document 1)). On the basis of these findings, the International Diabetes Federation published new guidelines for diabetes treatment (postprandial blood glucose control guidelines) in 2007, which recommend control of postprandial blood glucose levels as essential for Type 1 and 2 diabetic patients to alleviate diabetes and reduce risk of complications. As a practical step, an increased administration of an alpha-glucosidase inhibitor (voglibose) that is a drug for alleviating excessive postprandial blood glucose levels associated with diabetes, has been approved in Japan as a prophylactic agent against diabetes, aiming to “inhibit the development of Type 2 diabetes from impaired glucose tolerance”. As described above, there has been increasing awareness of the needs of nonpharmacological and pharmacological treatments against diabetes and borderline type diabetes, targeting the control of postprandial blood glucose levels in recent years.
Diabetes is treated mainly through diet regulation and exercise. When these fail to alleviate symptoms, pharmacological treatment is needed. Various types of drugs are available as prophylactic or therapeutic agents against diabetes. Among them, examples of insulin secretagogues include sulfonylurea agents (e.g., glibenclamide, glimepiride) and rapid-acting insulin secretagogues (e.g., mitiglinide), all of which stimulate beta cells of the pancreas so as to accelerate insulin secretion. These drugs are, however, known for their ineffectiveness (primary failure, secondary failure) and side effects such as induced hypoglycemic effects. Analogs (e.g., exenatide, liraglutide) of glucagon-like peptide-1 (GLP-1), which are hormones accelerating glucose-responsive insulin secretion in beta cells of the pancreas, have become available as novel insulin secretagogues, but they are administered by injection and known for their side effects of transient gastrointestinal tract disorders. Other examples of insulin secretagogues include dipeptidyl peptidase IV (DPP-IV) inhibitors (e.g., sitagliptin, vildagliptin), which inhibit the degradation of intrinsic GLP-1, but they are known for their side effects of epipharyngitis, headache, and infections. Alpha-glucosidase inhibitors (e.g., acarbose, voglibose) inhibit the degradation and digestion of carbohydrate and thus limit an abrupt increase in postprandial blood glucose levels, but they need to be taken immediately before meals and are known for their side effects such as distension and diarrhea and serious liver disorders. Biguanides (e.g., metformin, buformin) are insulin resistance improving agents enhancing insulin sensitivity and thereby alleviating hyperglycemia, but are known to potentially induce side effects such as lactic acidosis, nausea, and vomiting. Thiazolidinedione derivatives (e.g., pioglitazone, rosiglitazone) are peroxisome proliferator-activated receptor (PPAR) gamma agonists. The derivatives increase insulin sensitivity in adipose tissue, the liver, and skeletal muscles and thereby alleviate chronic hyperglycemia, but are known to tend to cause edema, weight gain, and serious side effects of liver disorders. Side effects of these drugs do not always occur, but remain as a major obstacle to high satisfaction with treatment. Therefore, the demand has been increasing for insulin secretagogues, particularly orally administrable insulin secretagogues, entailing few problems and side effects caused by conventional prophylactic and therapeutic agents as described above and inhibiting postprandial hyperglycemia without inducing hypoglycemia.
Fatty acid plays an important role in insulin use in the liver and skeletal muscles, glucose-responsive insulin secretion from the pancreas, and inflammation associated with fat accumulation in adipose tissue. A strong correlation is known between increased levels of fatty acid in blood plasma and the development of diabetes, metabolic syndrome, obesity, and adiposity.
GPR40, one of the G-protein-coupled receptors, is categorized in the free fatty acid receptor (FFAR) family and activated by C6-22 saturated or unsaturated fatty acid. It is reported that high expression of GPR40 is observed in beta cells of the pancreas where the receptor is involved in insulin secretion caused by fatty acid (Nature, 422, 173-176 (2003) (Non-Patent Document 2)). Non-fatty-acid low-molecular-weight compounds having a GPR40 agonist action have been found in recent years, and it is reported that thiazolidinediones, which are insulin sensitivity improving agents, and MEDICA 16, which is a hypolipidemic agent, also exhibit agonist actions (Biochem. Biophys. Res. Comm., 301, 406-410 (2003) (Non-Patent Document 3)).
In the pancreatic islets of Langerhans isolated from GPR40 knockout mice, the glucose-responsive insulin secretagogue action of fatty acid is lower than the case with normal mice. Accordingly, substances having a GPR40 agonist action like fatty acid are expected to have the effect of inhibiting postprandial hyperglycemia based on the glucose-responsive insulin secretagogue action in the pancreas. Therefore, substances having a GPR40 agonist action are considered to be effective as prophylactic and therapeutic agents against diabetes or borderline type diabetes.
In recent years, studies have been progressed on compounds having a GPR40 activating action as insulin secretagogues or therapeutic agents against diabetes. Technologies related to compounds having a GPR40 agonist action are disclosed, for example, in WO 2004/041266 pamphlet (Patent Document 1), WO 2005/086661 pamphlet (Patent Document 2), WO 2007/123225 pamphlet (Patent Document 3), WO 2008/001931 pamphlet (Patent Document 4), WO 2009/054390 pamphlet (Patent Document 5), WO 2009/054423 pamphlet (Patent Document 6), WO 2009/054479 pamphlet (Patent Document 7), WO 2011/046851 pamphlet (Patent Document 8), WO 2010/143733 pamphlet (Patent Document 9), WO 2007/033002 pamphlet (Patent Document 10), WO 2009/048527 pamphlet (Patent Document 11), WO 2009/111056 pamphlet (Patent Document 12), WO 2005/051890 pamphlet (Patent Document 13), WO 2004/022551 pamphlet (Patent Document 14), WO 2004/011446 pamphlet (Patent Document 15), WO 2008/030520 pamphlet (Patent Document 16), WO 2011/066183 pamphlet (Patent Document 17), WO 2010/091176 pamphlet (Patent Document 18), WO 2010/085525 pamphlet (Patent Document 19), WO 2009/039943 pamphlet (Patent Document 20), WO 2005/063729 pamphlet (Patent Document 21), and WO 2008/130514 pamphlet (Patent Document 22). These documents, however, do not disclose or suggest any compounds having a 3-hydroxy-5-arylisothiazolyl 1-oxide group.
A technique related to a compound having a 3-hydroxy-5-arylisothiazolyl group is disclosed in WO 2005/035551 pamphlet (Patent Document 23). The compound disclosed in Patent Document 23, however, is a compound having an inhibitory effect on protein tyrosine phosphatase 1B (PTP1B), and its structure is fundamentally different from that of the compounds according to the present invention. Another compound having a 3-hydroxy-5-arylisothiazolyl group is disclosed in WO 2000/042029 pamphlet (Patent Document 24). The compound disclosed in Patent Document 24, however, is a compound having an inhibitory effect on MAP kinase kinase (MEK) and containing a specific substituent on its side chain.
Quite recently, a compound with a GPR40 activating action having a 3-hydroxy-5-arylisoxazole group or a 3-hydroxy-5-arylisothiazole group is disclosed in WO 2011/052756 pamphlet (Patent Document 25) and WO 2011/078371 pamphlet (Patent Document 26).
In the development of drugs, various strict criteria must be met in terms of absorption, distribution, metabolism, excretion, and other factors as well as targeted pharmacological actions. There are various things to consider, for example, interaction with other drugs, desensitization or durability, digestive tract absorption after oral administration, speed to reach the small intestine, absorption speed and first pass effect, organ barriers, protein binding, drug metabolizing enzyme induction or inhibition, excretion route and clearance in the body, and application methods (application sites, methods, purposes). It is difficult to find a drug that meets all the criteria.
Several compounds are reported to have a GPR40 agonist action, but none of them has been marketed so far. Such agonists could also involve the above-mentioned general issues in the development phase of drugs. More specifically, they have problems in usefulness and safety, such as low metabolism stability and difficulty in systemic exposure by oral administration, unfavorable pharmacokinetic effects including absorption and persistence properties, an activity of inhibiting the human ether-a-go-go related gene (hERG) channel, possibly resulting in arrhythmia, and an activity of inducing, inhibiting drug metabolizing enzymes (e.g., cytochrome P450), or unwanted CNS-mediated side effects by brain penetration.
Therefore, required is a compound that solves these problems as much as possible and still has high efficacy.
In addition, required as a GPR40 agonist is a compound with fewer problems or side effects as described above than the aforementioned conventional drugs that have been used to prevent or treat diabetes (particularly Type 2 diabetes or borderline type diabetes).